1. Field of the Invention
The present invention is generally related to antennas and more particularly to an antenna for use in an antenna test system.
2. Description of the Related Art
The Cellular Telecommunications and Internet Association (CTIA) operates an equipment testing and certification program to ensure high-quality and reliability of cellular, personal communication services (PCS), enhanced specialized mobile radio (ESMR), and mobile satellite services products. Within this program, the CTIA establishes requirements for spherical-scanning antenna measurement systems (i.e., anechoic chambers). One challenge to antenna testing system designers is meeting the CTIA requirements while maintaining moderate range distances and ceiling heights. To meet the CTIA's antenna measurement system requirements efficiently, a measurement antenna that is low profile, dual-polarized, and multi-band is desired. The antenna preferably has a directive radiation pattern with high symmetry and low taper across the main beam, as well as low cross-polarization levels. It is also desirable that a single antenna assembly be capable of measuring in multiple bands and modes to increase throughput.
Spherical-scanning antenna test systems preferably use test-probe antennas that operate in a single mode of radiation for each desired polarization state and frequency band. Wideband horn antennas tend to change modes of operation over their range of frequencies and are thus not suitable for spherical-scanning antenna test systems. A properly designed probe antenna can provide this single mode of operation but only over a limited frequency band. Therefore, a multiplicity of probe antennas is necessary to cover all frequency bands. This is inconvenient and requires frequent changing of the probe antenna. It is therefore very desirable to have the widest possible band of operation and still maintain the single mode of radiation. It is also desirable to have multiple bands of operation on a single structure.
For spherical-scanning ranges, a probe antenna having a high profile in the direction of radiation will reduce the range distance and consequently degrade measurement certainty; thus, an antenna with a very low profile is desirable.
A stacked patch antenna is a good candidate for this application. Single-ended-fed (“single-fed”) stacked patch antennas are well-known in antenna literature as an approach for broad band or multi-band operation. However, to obtain bandwidths typically required for a spherical-scanning test system application, the patch antenna height is required to be large relative to the heights of single-fed patch antennas of the known art. At such height, a single-fed implementation suffers pattern asymmetry and increased cross-polarization. The high-frequency element of the antenna pattern is also more susceptible to diffraction/reflection effects from the low-frequency ground plane, which may cause ripple in the pattern peak. Such pattern ripple increases the difficulty in satisfying the CTIA requirements.